Electromyogram (cont.)
How is an intramuscular EMG done?
A needle is inserted through the skin into the muscle. The electrical activity is detected by
this needle (which serves as an electrode). The activity is displayed visually on an
oscilloscope and may also be detected audibly with a speaker.
Since skeletal muscles are often large, several needle electrodes may need to be placed at
various locations to obtain an informative EMG.
After placement of the electrode(s), the patient may be asked to contract the muscle (for
example, to bend the leg).
The presence,
size, and shape of the wave form (the action potential) produced on the
oscilloscope provide information about the ability of the muscle to respond to
nervous stimulation. Each muscle fiber that contracts produces an action potential. The size of
the muscle fiber affects the rate (how frequently an action potential occurs) and the size
(the amplitude) of the action potential.
How do you prepare for an intramuscular EMG?
For adults, no special preparation is needed. For infants and children, the physical and
psychological preparation depends on the child's age, behavior, and prior experience.
(For instance, has the child been traumatized by another medical or dental procedure?)
Does an EMG hurt?
Yes. There is some discomfort at the time the needle electrodes are inserted. They
feel like shots (intramuscular injections), although nothing is injected during an EMG.
Afterwards, the muscle may feel a little sore for up to a few days.
What other test is done during an intramuscular EMG?
A
nerve conduction velocity (NCV) test is often done at the same time as an EMG. In this test,
the nerve is electrically stimulated while a second electrode detects the electrical impulse
'down-stream' from the first. This is usually done with surface patch electrodes (they are
similar to those used for an electrocardiogram) that are
placed on the skin over the nerve at various locations. One electrode stimulates the nerve with
a very mild electrical impulse. The resulting electrical activity is recorded by the other
electrodes. The distance between electrodes and the time it takes for electrical impulses to
travel between electrodes are used to calculate the speed of impulse transmission (nerve
conduction velocity). A decreased speed of transmission indicates nerve disease.
The NCV test can be used to detect true nerve disorders (such as
neuropathy) or conditions
whereby muscles are affected by nerve injury (such as
carpal tunnel syndrome). Normal body
temperature must be maintained for the NCV test, because low body temperatures slow nerve
conduction.
Previous contributing authors and editors:
Medical Author: Frederick Hecht, MD, FAAP, FACMG
Medical Editor: Barbara
K. Hecht, Ph.D.
Last Editorial Review: 1/14/2009
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